Charged particle beams, laser beams, and neutral particle beams are used in a variety of microfabrication applications, such as fabrication of semiconductor circuitry and microelectromechanical assemblies. The term “microfabrication” is used to include creating and altering structures having dimensions of tens of microns or less, including nanofabrication processes. “Processing” a sample refers to the microfabrication of structures on that sample. As smaller and smaller structures are fabricated, it is necessary to direct the beam more precisely.
An aspect of semiconductor manufacturing that requires accurate beam positioning is the extraction of thin samples for transmission electron microscopy. Such samples are used for monitoring the semiconductor fabrication process. A thin, vertical sample, referred to as a lamella, is extracted to provide a vertical cross section of the structure.
A transmission electron microscope (TEM) allows an observer to image extremely small features, on the order of nanometers or smaller. In a TEM, a broad beam impacts the sample and electrons that are transmitted through the sample are focused to form an image of the sample.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the sample are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface. This technique can allow an observer to image features of sizes below one nanometer.
For both TEM and STEM, beam positioning while preparing the thin sample is important, because the beam must not etch away the feature of interest, yet the sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. Samples are typically less than 100 nm thick.
Therefore, accurate beam placement in electron microscopy and microfabrication applications is required. It has been found, however, that the impact point of a beam on a sample tends to drift over time. That is, when the operator instructs the system to position the beam at point P1, the beam actually ends up at position P2 a short time later. The difference between the coordinates of the points P1 and P2 is referred to as drift. The drift, including beam drift, stage drift, and sample drift, can be caused by thermal instabilities that cause slight movement of the stage on which the sample is positioned or of the elements that generate and focus the beam.
FIG. 1 shows a diagram of a prior art charged particle beam column used in a typical charged particle beam system for imaging or microfabrication. Charged particle source 102 provides charged particles, such as electrons or ions, that are used to form the beam. Condenser lens system 104 draws the charged particles from source 102, forms the charged particles into beam 103, and directs beam 103 toward the sample 113. Sample 113 is held in a fixed position in sample chamber 115 of column 100 by sample holder 112 and stage 110. Objective lens system 106 produces an image of sample 113 based upon the interaction of the charged particles in beam 103 with sample 113. In some charged particle systems, such as a TEM, some of the charged particles in beam 3 pass through sample 113 and are projected by projecting lens system 114 onto viewing device 116 at the bottom of column 100.
FIG. 2 shows a cross section view of an objective lens system and a sample chamber of a prior art charged particle beam column. The objective lens 106 comprises a plurality of coils 202a-202c that form an electromagnet around the axis of charged particle beam 103. Coils 202a-202c of objective lens 106 are often positioned in close proximity to the sample chamber 115. When the temperature of sample chamber 115 varies, the position of stage 110 and sample holder 112 changes relative to the poles of the electromagnet, which causes thermal drift. This thermal drift is mainly caused by coils 202a-202c of objective lens 106 being located close to sample chamber 115. Coils 202a-202c carry electric current to produce the magnetic field by which objective lens 106 operates. If the magnification of the system changes, then the amount of ampere-turns changes, which leads to a change in power dissipation and thereby a change in coil-temperature. The coil is typically mounted in close proximity to sample chamber 115 and thermal conduction will cause the temperature of sample chamber 115 to change. While the drift may be small, drift becomes more significant as smaller structures are fabricated or imaged.
One approach to preventing thermal drift is to reduce the heat transfer between the objective coils and the specimen-chamber by insulating them from each other with a layer of stagnant air, which can be a good thermal insulator. But even a layer of stagnant air is not enough to prevent drift. Other approaches have included using constant power lenses. But using constant power lenses results in extra power dissipation, oversized coils, spooling of energy and coolwater, and expensive, extremely stable power supplies capable of providing a voltage with a error of less than one part-per-million (“PPM”). For example, a 1000 Volt power supply with an error of 1 PPM (i.e., 0.001 Volt) provides a steady voltage between 999.999 Volts and 1000.001 Volts.
Another approach to compensate for thermal drift is changing the flow of water or coolant to control the temperature of the coil. However, coil temperature is not uniform, or to be more precise the outside coil temperature varies. As a result, changing the flow of water or coolant to control the temperature of the coil has not been effective at reducing thermal drift.
Yet another approach to compensate for thermal drift is using beam deflection. As the charged particle beam begins to drift, the charged particle beam can be deflected by deflecting plates or lenses within the beam column to move the beam back into proper registration. However, deflecting the beam to compensate for thermal drift reduces the performance of the beam. A better approach would be to reduce or eliminate the cause of thermal drift instead of compensating for it.